1. Field of the Invention
The present invention relates to position sensors generally. The invention has particular although not exclusive relevance to non-contact linear and rotary position encoders. The invention is particularly suited for use in systems where the object whose position is being sensed can be tilted relative to the measurement direction.
2. Related Art
Many types of non-contact linear and rotary position encoders have been proposed for generating signals indicative of the position of two relatively moveable members. Typically, one of the members carries one or more sensor coils and the other carries one or more magnetic field generators. The magnetic field generators and the sensor coils are arranged such that the amount of magnetic coupling between them varies as a function of the relative position between the two members. This can be achieved by, for example, designing the sensor coils so that their sensitivity to magnetic field varies in a predetermined manner along the measurement path. Alternatively, the magnetic field generators can be designed so that the magnetic field which they generate varies in a predetermined manner along the measurement path.
One example of this type of position encoder is the Inductosyn, which comprises a contactless slider which is arranged to detect the field generated by a stationary track, or vice versa. The stationary track comprises a repeating pattern of conductors which generates a magnetic field of substantially sinusoidal variation in the measurement direction when a current is applied to them. This magnetic field is detected by the moving slider, which comprises sin and cos detector tracks. The position of the two relatively moveable members is then determined from the spatial phase of the signals detected by these two detector tracks.
The applicant has proposed in its earlier International Application WO95/31696, a similar type of position encoder in which one member carries an excitation coil and a number of sensor coils and the other member carries a resonator. In operation, the excitation coil energises the resonator which in turn induces signals in the sensor coils which sinusoidally vary with the relative position between the two members. A similar system is disclosed in EP 0182085 which uses a conductive screen in place of the resonator. However, the use of the conductive screen in place of the resonator has the disadvantages that the output signal levels are much smaller and that the system cannot be operated in a pulse-echo mode of operation, in which a short burst of excitation current is applied to the excitation winding and then, after the excitation current has ended, detecting and processing the signals induced in the sensor coils.
A problem common to all of these known position sensors is that a positional error is introduced into the measurements if the moveable member is tilted relative to the other member. In some applications, such as machine tool applications, it is possible to physically restrict the movement of the two relatively moveable members, e.g. by using guide rails or the like. However, sometimes this is not possible. For example, in an X-Y digitising tablet, such as the one described in U.S. Pat. No. 4,848,496, the moveable member (the stylus) is moved by a human operator and its tilt relative to the tablet varies considerably during normal use.
Most digitising tablets which have been proposed to date employ a large number of overlapping but separate excitation and sense coils which are spread over the active area of the digitising tablet. The system identifies the current position of the stylus by detecting the excitation and sensor coil combination, which provides the greatest output signal levels. Some systems, such as the one disclosed in U.S. Pat. No. 4,848,496 mentioned above, perform a quadratic type interpolation to try to determine more accurately the current position of the stylus. However, this type of system suffers from the problem that it requires a large number of excitation coils, which must be individually energised, and a large number of sensor coils, which must be individually monitored for each energised excitation coil. There is therefore a trade off between the system's response time and the accuracy of the tablet. In particular, for high accuracy, a large number of excitation and sense coils are required, however, as the number of excitation coils and sensor coils increases, the system's response time decreases. The number of excitation and sense coils used in a given system is therefore governed by the required application.
EP-A-0680009 discloses such a digitising tablet system which is also arranged to process the signals from the different sensor coils in order to determine the orientation of the stylus in the X-Y plane.